This invention relates to a multilayer ceramic substrate, and more particularly, to metal layers of the multilayer ceramic substrate, wherein the conductive and insulating layers of the multilayer ceramic substrate are applied as thick film pastes, and each layer after it is applied is fired to secure the layers to each other and to the ceramic substrate.
Multilayer ceramic substrates (or multilayer ceramic carriers) are used to mount and interconnect a plurality of integrated circuit chips. Each layer of a multilayer ceramic substrate may be a dielectric, or insulating layer, or a metallic or conductive layer, the pattern of each layer being predetermined such that desired interconnections between integrated circuit chips are made, and electrically isolated from undesired connections. Each layer of the multilayer ceramic substrate is produced using a thick film paste deposited on the surface of the substrate, followed by a firing in a furnace, such as the furnace described in the related patent application referred to above. As described in the related patent application, the firing of each layer of thick film paste produces gases and other undesirable residue material which are extracted during the firing process. However, sometimes not all the undesirable residual gases may be extracted during the first firing, but are subsequently released and extracted on subsequent firings. These undesirable residual gases can only escape from within the layers if the material of the newly added layer after being fired is sufficiently porous to provide paths which allow the undesirable residual gases from lower layers to escape. Since sintered metal is not sufficiently porous to allow any undesirable residual gases to penetrate through the sintered metal layer to escape, when residual gases are produced, the likelihood is high that a higher or a top layer of fired, or sintered, metal will delaminate (or blister) from the layer immediately below due to the pressure exerted by the residual gases against such a layer of sintered metal. This localized loss of adhesion of a fired metallic layer to a fired dielectric layer is a source of reliability problems and results in rejecting the substrate. This rejection is usually done late in the manufacturing process of the multilayer ceramic substrate in which the substrate has gone through many firings and included many hours of labor thereby rejecting a high cost part.
Thus, there exists a need to eliminate the blistering/delamination problem by providing a structure which allows the undersirable residual gases which are not extracted during a first firing to escape during subsequent firings.